DPI / PPI
CreativePPI (pixels per inch) and DPI (dots per inch) both describe density per inch: PPI is a digital image's pixel density at a given print size, DPI is how many ink dots a printer lays down. For screen ads only the pixel dimensions matter and the PPI tag is ignored; for print, 300 PPI is the standard.
Example
Set artwork to 300 PPI for a printed flyer; for a 300×250 web banner the PPI value is ignored.